Применение математического моделирования в аддитивных технологиях для топологической оптимизации

The paper deals with the questions of introducing the additive technology of selective laser melting (SLM) into the production of the main structural elements of a combustion chamber for ground-based gas pumping, power and transport gas turbines. A fundamentally new design of a low-emission two-circuit burner was developed, adapted for the manufacture of products with a complex geometry through the use of selective laser melting (SLM) of metal powders according to mathematical CAD-models with the help of an advanced production technology. Technical difficulties associated with the implementation of additive manufacturing technology were overcome. The process chain was tried out from design to manufacturing and quality control of finished structural elements under conditions of mass production. The deficiencies of the parts manufactured by the additive technology were revealed and ultimately eliminated. Successful development of burners for a low-emission combustion chamber using a new technology significantly reduces labor intensity and time of production while maintaining high quality.

In the Laser Powder Bed Fusion (L-PBF) process, 3D components with complex geometries are fabricated in a layer-by-layer fashion by using a controlled laser beam to selectively melt particular regions of the metal powder bed. However, due to the stochastic nature of the L-PBF process, the top surface roughness of each solidified layer tends to be different even when the optimal processing conditions for the different positions on the build plate are employed. As a result, the mechanical properties of the built components frequently vary from one component to the next. Accordingly, this study proposes an Intelligent Additive Manufacturing Architecture (IAMA) for controlling the surface roughness of each build layer through an appropriate adjustment of the laser re-melting parameters. The IAMA architecture comprises five modules, namely In-Situ Metrology (ISM), Ex-Situ Metrology (ESM), Automatic Virtual Metrology (AVM), Additive Manufacturing Simulation (AMS) and Intelligent Compensator (IC). The feasibility of the proposed architecture is demonstrated by comparing the top surface roughness of cubic and mechanical strengths of tensile test samples built using the proposed method with those built using a traditional L-PBF approach without surface roughness control. It is found that the samples fabricated using the IAMA approach have an average top surface roughness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.6~\mu \text{m}$ </tex-math></inline-formula> and a standard deviation is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.7~\mu \text{m}$ </tex-math></inline-formula> . By contrast, the samples produced using the traditional L-PBF approach have an average surface roughness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$13.45~\mu \text{m}$ </tex-math></inline-formula> and a standard deviation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5~\mu \text{m}$ </tex-math></inline-formula> . In addition, the specimens produced with the assistance of IAMA architecture have an average tensile strength of 1013 MPa with a standard deviation of 69.5 MPa, while those printed without surface roughness control have an average tensile strength of 903 MPa with a standard deviation of 101.4 MPa Note to Practitioners—As L-PBF produce part in a layer-by-layer manner, therefore, the roughness on the top surface of previous layer have a strong influence on the printing quality of current layer. The variations of surface roughness will lead to the fluctuation of mechanical properties of the fabricated components. Additionally, to the best of author’s knowledge, current commercial L-PBF machines can not actively control the surface roughness of samples during the process. The proposed IAMA in this work can predict and control the roughness on the top surface of each layer during L-PBF process. Therefore, the constructed architecture has strong potential for integrating into the commercial L-PBF machine for controlling the surface roughness on the top of the parts during the manufacturing process. As a result, the quality of the fabricated components is expected to be in consistent. Accordingly, L-PBF machine equipped with IAMA will have a strong potential in applying for mass production of the components in aerospace and automobile industries.

Selective laser melting (SLM) is one of the Additive manufacturing (AM) processes that can build physical part in an added material method from digital data. In such a process, computer designed part model will be decomposed into hundreds of thousands of layers. The layered information is then transferred to SLM equipment and the part is built in a layer by layer fashion. Each powder layer will be scanned and melted in the required region by a high energy laser beam in a given scanning pattern so as to form a desired geometry. Finally, fully functional parts can be produced by repeatedly powder deposition, melting and solidification process. This process offers numerous advantages such as tooling-free productions and design freedom in geometry. In addition, SLM process is quite suitable for complicated parts such as customer designed medical implants and internal channels which are difficult to manufacture by conventional methods such as casting and machining. However, the localized heating and cooling process can lead to defects such as high residual stress, part distortion or delamination failure in SLM fabricated parts. These potential defects may impede the wide application of this technology. It is known that the laser beam scanning path will affect the thermomechanical behaviors of the build part, and thus, altering the scanning pattern may be a feasible strategy to reduce residual stresses and deformations by influencing the heat intensity input distribution. In this study, a 3D sequentially coupled finite element method (FEM) model, incorporating a volumetric moving Gaussian heat source, powder as well as solid material temperature dependent properties and layer addition features, was developed to study the complex thermomechanical process of SLM. The model was applied to evaluate six different scanning strategies effect on part temperature, stress and deformation. The major results have been summarized as follows. (1) Among all cases tested, the out-in scanning pattern has the maximum stresses along the X and Y directions; while the 45 degree inclined scanning may reduce residual stresses in both directions. (2) Large directional stress difference can be caused by back and forth line scanning strategy while minor directional stress difference is observed for other tested cases. (3) X and Y directional stress concentration is shown around the edge of deposited layers and the interface between deposited layers and substrate for all cases. (4) The 45 degree inclined scanning case has the smallest build direction deformation while the in-out scanning case has the largest deformation among the tested cases.

Digital twins for additive manufacturing (AM) have drawn much research attention recently, thanks to advancements in artificial intelligence and machine learning. Machine learning takes the process and measurement data from the manufacturing process to build data-driven models instead of physics-based descriptive models. The latter are usually hard to obtain for complex AM processes such as laser powder bed fusion. This study proposed a digital twin framework for the laser powder bed fusion AM process control and optimization. The framework is created based on the recently developed advanced point-wise scan control method. It consists of four components: digital twin of process design, digital twin of process control, digital twin of process monitoring, and digital twin of printed part. Their construction is detailed, and potential applications are demonstrated/discussed.

Additive manufacturing methods, such as the laser powder bed fusion, do not need any special tool or casting mold. This enables the fast realization of complex and individual geometries with integrated functions. However, the local heat input during the manufacturing process often leads to residual stresses and distortion. This in turn causes poor quality, scrap parts or can even terminate a job prematurely if the powder recoating mechanism collides with a distorted part during the process. This study investigates the generation mechanisms of residual stresses and distortion during laser powder bed fusion (LPBF) of stainless steel 316L in order to reduce these effects and thus contribute to improved process safety and efficiency. Therefore, numerical investigations with a finite element model on the scale of a few melt tracks and layers serve to develop a detailed understanding of the mechanisms during production. The work includes an investigation of the build plate temperature, the laser power and speed and the layer thickness. The results show a strong dependency on the build plate preheating and energy per unit length. A higher build plate temperature and a reduction of the energy per unit length both lead to lower residual stresses.

Smooth and notch fatigue behavior of selectively laser melted Inconel 718 with as-built surfaces

A binary Nickel-Titanium shape memory alloy was processed by Laser Powder Bed Fusion (LPBF) to manufacture lattice structures. The chemical composition of the powder was Ni 50.9 Ti 49.1 (at%) with the aim of obtaining pseudoelastic properties in the printed part. Adapted scanning strategies were chosen and used on a standard LPBF machine to achieve complex geometries with small, uniform feature sizes and a more precise modification of the local energy input. The as-built structures were analyzed with optical, thermal, and mechanical methods by means of optical microscopy, differential scanning calorimetry, and compression tests. The results demonstrate that LPBF combined with adapted scanning strategies can generate complex and homogeneous NiTi geometries (like metamaterials and programmable materials). Furthermore, the results show that scanning strategies have a significant influence on the thermal and therefore mechanical properties of the structures. We conclude that adapted scan strategies overcome the limitations of ordinary contour-hatch scan strategies and lead to shape memory properties which cannot be realized with conventional manufacturing techniques.

Selective laser melting (SLM) can be used to manufacture functional metal parts with complex geometries that cannot be produced by traditional manufacturing methods. However, SLM process control cannot yet guarantee the end part quality required for critical applications. The application of model-based control strategies to SLM is complicated by both the closed architecture of industrial SLM machines and the lack of suitable control-oriented process models. In this paper we (1) present an open-source SLM printer that allows implementation of the on-the-fly power adjustment and (2) use a data-driven method, iterative learning control (ILC) to learn the suitable laser power profile using the melt pool emission measurements from a coaxial camera. We demonstrate the effectiveness of the proposed ILC approach through experiments on the open-source SLM machine.

Microstructure and fatigue performance of SLM-fabricated Ti6Al4V alloy after different stress-relief heat treatments

Additive technologies, in particular selective laser melting (SLM), enable to manufacture the products with complex geo­metries. The SLM technique can help to effectively expand the titanium nickelide scope of application. However, SLM is a complex process – numerous factors significantly affect the characteristics of the resulting alloy. When the SLM technique is used, as the material is subject to laser processing, the content of nickel in the alloy drops due to evaporation, which can lead to changes in the tempe­ratures of martensitic transformations. This impact on the resulting alloy characteristics can be regulated by changing the para­meters of the SLM process. The objective of our research was to develop the processing methods for manufacturing samples from two commercial TiNi alloy powders using the SLM technique and to analyze the factors causing defects in the obtained samples. At the same time, processing methods with low values of volumetric energy density were used to reduce possible evaporation of nickel during printing. The initial powders were examined for the presence of impurities or other factors affecting the quality of the manufactured samples. The processing method A4 that we have developed for powder 1 enables to obtain a defect-free sample with the density of 6.45 g/cm3. It was found that none of the processing methods used enabled to obtain a defect-free sample from powder 2 due to presence of a large amount of oxygen impurities, including in particular Ti4Ni2Oх secondary phase, which leads to embrittlement and destruction of the samples. Therefore, high content of oxygen in the initial powders has a negative impact on the quality of the samples manufactured using the SLM technique.

The review analyses the microstructure of the commercial Co–Cr–(Mo, W) dental alloys fabricated by 3D digital selective laser melting (SLM), which is the most promising technique among the emerging additive fabrication technologies used for metal products manufacturing in dentistry. In this regard, the main goal is to compare the microstructures of the metal dental products produced by two currently used technologies, namely, conventional casting and SLM. We consider the latest research published from 2013 to 2022. The microstructures are evaluated using optical microscopy (OM), scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM–EDS), x-ray diffractometry (XRD), electron backscatter diffraction (EBSD) pattern analysis, and atomic force microscopy (AFM). The microstructure analysis allows concluding whether the SLM fabrication process is suitable for dental applications. As shown, the microstructure of the Co–Cr dental alloys depends on both the chemical composition of the samples and the parameters of the manufacturing technique used. Experimental results have proven that, in contrast to the conventional casting, the SLM-fabricated specimens display superior microstructure due to complete local melting and rapid solidification. Additionally, the SLM process minimizes residual flaws and porosity. As a result, SLM allows producing the dense material comprising homogeneous fine-grain microstructure.

The distinct deposition method of selective laser melting (SLM) enables the near‐net‐shaping of high‐speed steel tools with complex geometries and integrated functions, such as adapted cooling channels, which offers economic advantages and has, thus, attracted considerable attention. However, SLM‐processed high‐speed steels are prone to cracking due to their high carbon and alloying element contents and high internal stresses. Varying the steel composition to stabilize austenite can help to reduce the residual stresses. Herein, the effect of nitrogen atmosphere on the densification behavior, microstructural evolution, and mechanical properties of high‐speed steel in the SLM process is investigated. The results demonstrate that nitrogen can dissolve in the steel. In combination with suitable SLM parameters, this leads to a fully austenitic microstructure with no cracks. After heat treatment, the microstructure of steel transforms into tempered martensite, along with the precipitation of tiny V‐rich M(C,N) carbonitride and V‐rich MC carbide. The hardness and bending strength of the tempered sample reach the highest of 61.3 HRC and 3659 MPa, respectively. Its abrasive resistance is also improved. An alloy design idea is provided that is based on the reactive atmosphere of high‐speed steel and other materials for processing by SLM.

Ultrasonic testing of thick and thin Inconel 625 alloys manufactured by laser powder bed fusion

Laser powder bed fusion (LPBF) provides a rapid and versatile approach for producing parts with complex geometries. However, many parts with intricate geometries have overhang structures, which are not easily fabricated by using LPBF and are often downgraded by staircase effects, warpage, cracks, and dross formation. Thus, the present study proposes a combined numerical and experimental approach for determining the optimal settings of the laser power and scanning speed that minimize the surface roughness and maximize the density of Inconel 718 LPBF overhang structures. In the proposed approach, the heat transfer simulations are employed to determine the melt pool depth, the melt pool length, and the solid cooling rate within the feasible input space of laser power and scanning speed combinations. Notably, the simulations take account of both the difference in the material properties of the solid and powder materials, respectively, and the variation of the laser absorptivity in the depth direction of the powder layer. The simulation results are then used to train artificial neural networks for predicting the melt pool depth for 3600 combinations of the laser power and scanning speed within the input space. The resulting processing maps are screened in accordance with three quality criteria (namely the melt pool depth, the melt pool length, and the solid cooling rate) to determine the optimal processing region, which improves the surface roughness. The feasibility of the proposed approach is demonstrated by fabricating 10 × 10 and 20 × 20 mm2 horizontal overhang structures using parameter settings chosen from the optimal processing map. It shows that the optimal processing conditions result in a low surface roughness and a maximum density of 99.78%.

A lightweight Fe–Mn–Al–C austenitic steel with ultra-high strength and ductility fabricated via laser powder bed fusion